1. Field of the Invention
The present invention relates to a control technique for an optical modulator used in optical communications, and in particular, relates to a control technique for an optical modulator suitable for generating signal light corresponding to the Carrier-Suppressed RZ (Carrier Suppressed Return-to-Zero: to be referred to as CS-RZ below) modulation method.
2. Description of the Background Art
At present, optical transmission systems in which optical signals are transmitted at speeds of around 10 Gb/s are beginning to be in practical use, but due to the recent rapid increase in network usage, further increases in network capacity are sought, and in addition, demand for implementation over even longer distances is increasing.
In optical transmission systems with transmission speeds of 10 Gb/s or more, because the affect of wavelength dispersion on the waveforms is large and the optical spectrum is broadened, WDM transmission in which channel lights are arranged with a high level of density is difficult. Particularly, in 40 Gb/s optical transmission systems, wavelength dispersion is one of factors limiting the transmission distance.
Dispersion compensation technology in which a dispersion amount in the optical transmission path is accurately measured to compensate has been investigated as a method of solving the problems described above (Japanese Unexamined Patent Publication No. 11-72761 and Japanese Unexamined Patent Publication No. 2002-077053). Furthermore, in order to realize such an optical transmission system as described above, the development of a modulation method with an even slightly higher dispersion tolerance is essential. Specifically, in order to achieve a long distance optical transmission system, a modulation method in which an excellent optical S/N ratio can be ensured, in other words, a modulation method which is resistant to the self phase modulation (SPM) effect and for which the upper limit of the power of optical input to the optical transmission path can be made high, is required. In addition, in order to increase capacity, a modulation method with a narrow optical spectrum allowing high density WDM optical transmission is required.
Recently, research has been conducted into new modulation methods such as the carrier suppressed RZ (CS-RZ) modulation method (for example, Y Miyamoto et. al., xe2x80x9c320 Gbit/s (8xc3x9740 Gbit/s) WDM transmission over 367 km zero-dispersion-flattened line with 120 km repeater spacing using carrier-suppressed return-to-zero pulse formatxe2x80x9d, OAA""99 PD, PdP4 and the like). An advantage of this CS-RZ modulation method is that because, as described below, the optical spectrum width is ⅔ times that of the RZ modulation method, the waveform dispersion tolerance is broad, and the high density channel arrangement in WDM is possible. Furthermore, because waveform degradation due to the self phase modulation (SPM) effect is minimal, it becomes possible to ensure an optical S/N ratio suitable for long distance transmission.
FIG. 14 is a diagram showing a basic structure for generating a 40 Gb/s CS-RZ modulation signal.
In FIG. 14, a light source 100 generates continuous light. The continuous light output from this light source 100 is input, in sequence, to two LiNbO3 modulators 110 and 120 (to be referred to as LN modulators below) connected in series, to thereby be modulated.
For example, a data signal, generated in a data signal generating section 111, with a bit rate of 40 Gb/s and-corresponding to an NRZ modulation method is applied to a signal electrode (not shown in the figure) of the former LN modulator as a drive signal, and as a result, the former LN modulator 110 modulates the continuous light from the light source 100 according to the data signal, and outputs a 40 Gb/s NRZ signal light having a waveform as illustrated in (a) of FIG. 15 to the latter LN modulator 120.
As the latter LN modulator 120, a Mach-Zehnder (MZ) modulator with two signal electrodes is used, for example. The latter LN modulator 120 further modulates the NRZ signal light received from the former LN modulator 110 as a result that a first drive signal and a second drive signal generated based on a clock signal with a frequency of xc2xd times the bit rate of the data signal are applied to the respective signal electrodes thereof, and outputs a 40 Gb/s CS-RZ signal light having a waveform as illustrated in (b) of FIG. 15. Here, a clock signal with a frequency of 20 GHz which has a waveform such as a sine wave is generated in a clock signal generator 121, and after being split into two in a splitter 124, the split clock signals are adjusted in the phase shifters 125A, 125B, respectively so that a phase difference between the split clock signals is approximately 180xc2x0, and further, respective amplitudes of the clock signals are adjusted in amplifiers 126A and 126B, respectively, to become the first and second drive signals to be applied to each signal electrode of the LN modulator 120.
Furthermore, a part of the clock signal generated in the clock signal generator 121 is split in a splitter 122 and sent to the data signal generating section 111, and the phase difference between each signal is controlled by adjusting the phase of the clock signal by a phase shifter 123, so that phases of the data signal and the clock signal are synchronized.
Here, the principle of generating a 40 Gb/s CS-RZ signal light is described simply using the optical intensity characteristics relative to the drive voltage of the LN modulator, as shown in FIG. 16.
Generally, when generating a signal light corresponding to the NRZ modulation method or the RZ modulation method, using an optical modulator in which the optical intensity characteristics varies periodically relative to the drive voltage, modulation is performed by supplying, to the optical modulator, a drive voltage (hereafter, this drive voltage is referred to as Vxcfx80) which corresponds to the xe2x80x9cpeaks and valleysxe2x80x9d or the xe2x80x9cvalleys and peaksxe2x80x9d which adjoin each other in the optical intensity characteristics. Here, the xe2x80x9cpeakxe2x80x9d of the optical intensity characteristics refers to the apex of light emission, and the xe2x80x98valleyxe2x80x99 refers to the apex of light extinction.
On the other hand, when generating signal light corresponding to the CS-RZ modulation method, the 40 Gb/s NRZ signal light which was modulated in accordance with the data signal in the former LN modulator 110 shown in FIG. 14, is further modulated in the latter LN modulator 120, according to a 20 GHz clock signal with a frequency of xc2xd times the bit rate of the data signal. And as shown in the left of FIG. 16, a drive voltage (hereafter, this drive voltage is referred to as 2Vxcfx80) which corresponds to the xe2x80x9cpeak, valley, peakxe2x80x9d of the optical intensity characteristics relative to the drive voltage is supplied to this latter LN modulator 120. This modulation of light is performed with each level xe2x88x921, 0, 1 of the clock signal corresponding respectively to an on, off, on state of the light, and the generated CS-RZ signal light becomes a binary optical waveform as shown on the right of FIG. 16. For the signal light in this CS-RZ modulation method, because the optical phase of each bit has a value of either 0 or xcfx80, then as shown in the calculation results of the optical spectrum in FIG. 17, the carrier component of the optical spectrum is suppressed in comparison with the signal light in the RZ modulation method.
As seen in the results of experiments on the optical spectrum and the optical waveform shown in FIG. 18, for example, the signal light in the CS-RZ modulation method generated in the manner described above has an optical waveform approximately equal in shape to the optical waveform obtained by the RZ modulation method, but the optical spectrum width is narrower than that in the RZ modulation method. Furthermore, as seen in the results of experiments relating to wavelength dispersion tolerance shown in FIG. 19, for example, the range of total wavelength dispersion for which the power penalty is 1 dB or less, is approximately 40 ps/nm in the case of the RZ modulation method, and is approximately 50 ps/nm in the case of the CS-RZ modulation method. It is apparent that a dispersion tolerance of the signal light in the CS-RZ modulation method is expanded compared with that of the signal light in the RZ modulation method.
However, although the signal light corresponding to the CS-RZ modulation method has the advantages described above, there are also disadvantages in that the phase difference between the first and second drive signals, which are applied to the latter optical modulator, and are driven based on the clock signal, must be precisely adjusted, and that the phase difference between the data signal and the clock signal, which are used to drive the former optical modulator must also be precisely adjusted. In addition, because there is a possibility that a phase shift may occur due to environmental variation such as variations in temperature, phase variation in each signal must be detected to perform a feedback control, during system operation.
However, no specific technology relating to the detection of phase shift between the drive signals, and the feedback control for an optical modulator for use with the CS-RZ modulation method has yet been proposed.
In view of the above circumstances, an object of the present invention is to provide a control technique for an optical modulator, which can accurately detect a phase shift between signals of drive system of the optical modulator, and can control a phase difference between the drive signals so that optimal drive conditions can be obtained in a stable manner.
In order to achieve the aforementioned object, according to one aspect of the present invention, a control apparatus for an optical modulator which generates a signal light corresponding to the CS-RZ modulation method, comprises: a monitor section that extracts a specific frequency component from an optical spectrum of signal light output from the optical modulator to detect the optical intensity thereof; and a control section that determines a phase shift between a plurality of drive signals supplied to the optical modulator based on the optical intensity detected by the monitor section, and controls a phase difference between the drive signals so that the phase shift is minimized.
In this construction, the optical intensity of the specific frequency component of the CS-RZ signal light output from the optical modulator is detected by the monitor section, and based on this optical intensity, the phase shift between the plurality of drive signals supplied to the optical modulator is determined by the control section, and the phase difference between the drive signals is then optimized so that the phase shift is minimized. Consequently, it becomes possible to generate a CS-RZ signal light under stable drive conditions.
Furthermore, in the aforementioned control apparatus for an optical modulator, the optical modulator includes a data side optical modulation section to which a drive signal corresponding to a data signal is supplied, and a clock side optical modulation section to which at least two drive signals corresponding to a clock signal with a frequency of xc2xd times the bit rate of the data signal are supplied, and the control section may determine, based on the optical intensity detected in the monitor section, at least one of a phase shift between the drive signals supplied to the clock side optical modulation section and a phase shift between the data signal and the clock signal, to feedback control a phase difference between the these signals so that the phase shift is minimized.
Consequently, the feedback control of the phase shift between the drive signals supplied to the clock side optical modulator, the feedback control of the phase shift between the data signal and the clock signal, or the feedback control of the phase shifts is performed by the control section.
Another aspect of a control apparatus for an optical modulator according to the present invention comprises: an optical modulator incorporating a section for branching an optical waveguide into a first branch optical waveguide and a second branch optical waveguide, and a section for combining these first and second branch optical waveguides, that controls the refractive indexes of the first and second branch optical waveguides using a first electrode and a second electrode which are provided in the first and second branch optical waveguides respectively, to obtain periodic optical intensity characteristics corresponding to a difference between the refractive indexes; a drive circuit that applies a voltage to the first and second electrodes so that a modulation operation is performed based on one cycle of the optical intensity characteristics of the optical modulator; a phase controller that controls the phase of this drive circuit; and a detector that detects a specific optical wavelength component in an output of the optical modulator, wherein the phase of the drive circuit is controlled by the phase controller based on the detection result from the detector.
In this construction, a specific optical wavelength component of the signal light output from the optical modulator is detected by the detector, and based on this detection result, the phase shift between voltage signals applied to the first and second electrodes of the optical modulator is controlled by the phase controller. Consequently, it becomes possible to generate, under stable drive conditions, a signal light modulated based on one cycle of the optical intensity characteristics of the optical modulator.
Other objects, characteristics and advantages of the present invention will become apparent from the following description of the embodiments in relation to the appended drawings.